1. Field of the Invention
The present invention relates to level sensing and, more particularly to electro-optical devices for measuring the distance between a reference level and a variable surface, such as the surface level of liquid in a vessel.
2. Description of the Prior Art
A number of non-intrusive, optical level gauging techniques have been developed. Several of these common methods of distance measurement are based on the speed of light. One such method of deriving distance from the speed of light is to transmit an amplitude-modulated light beam and to compare the phase of the return signal with the phase of the outgoing signal. The relative phase of the two signals depends on the distance to the subject and the frequency of the modulation used. Such a technique is disclosed, for example, in U.S. Pat. No. 5,194,747. However, the accuracy of the phase comparison method is limited by the ability of the phase detector used to resolve phase and the amount of isolation that can be obtained between the incoming and outgoing signals.
Another type of distance measurement technique, disclosed in U.S. Pat. No. 5,309,212, measures distance by creating an oscillator with a source of projected energy and a sensor to detect this energy after it has traveled some distance. The detected signal is amplified and applied to the energy source to form a feedback loop which oscillates at a frequency which depends on the distance the energy travels and the speed at which it travels. The frequency can be measured, and either distance or speed determined if the other is known. This prior art technique produces a narrow optical beam that retains a diameter of less than a centimeter out to a distance of ten meters or so. It has been found to be very difficult to get such a beam lined up on a liquid surface so that a detectable reflection comes back into the detector.